Mobile machines such as elevating platforms, truck cranes, concrete pumps, for example, have extendable parts provided thereon, for example telescope cylinders or extendable supports. FIG. 1 shows an example of such a known mobile machine in the form of a mobile crane 100. In the example shown, the mobile crane 100 includes four supports, only two of which can be seen in FIG. 1 and are designated by reference numerals 102 and 104 in the FIG. Each of said supports 102, 104 includes a horizontal punch 102a, 104a, each of which has a vertical punch 102b, 104b arranged thereon. The horizontal punches 102a and 104a are movable in the horizontal direction and may be extended, as is shown in FIG. 1, FIG. 1 indicating an extension length L. The vertical punches 102b and 104b are arranged to be vertically movable. The mobile crane 100 further includes a pivoted crane top piece 106 comprising a crane arm 108. The crane arm 108 may include several segments not shown in FIG. 1 which may be extended with regard to the representation shown in FIG. 1 so as to adjust a height and range of the crane in accordance with the requirements. The crane arm 108 or the segments of the crane arm 108 may be actuated by corresponding hydraulic telescope cylinders, for example.
In mobile machines, the horizontal arms 102a, 104a are extended for support purposes, and the vertical punches 102b, 104b arranged on their outer ends are moved downward to enable the vehicle or the mobile machine to be supported on the ground, so that the vehicle takes on a stable position and no longer rests on the wheels, for example. Depending on the circumstances, the crane arm 108 may be extended to corresponding heights during operation, as was mentioned above, and a tilt of the crane arm 108 is adjusted, for example, by retracting/extending a cylinder 110.
Both actuation of the supports 102, 104 and actuation of the cylinders in the crane arm 108 and/or of the cylinder 110 are to be monitored in order to insure safe operation of the crane.
The further the horizontal punches 102a, 104a are extended, the larger the extent of lateral cantilevering of the work tool, e.g. of the crane arm 108 or of the lifting platform, may be without there being a danger of tipping or of instabilities. A simple control would consist in invariably extending the supports to a maximum in order to enable as large a deflection of the work tool as possible. However, this maximum extension of the supports is not always possible or not always desired, for example due to limited spatial conditions or due to the fact that a maximum deflection of the work tool is not required at all. For this reason it is desirable to measure in as exact a manner as possible the extent to which the four extension punches 102a, 104a and/or the cylinders are extended for actuating the work tool so as to then ensure optimum stability, in connection with load measurement, while making optimum use of lateral cantilevering. To achieve a sufficiently high level of work safety, it is useful that this length measurement be effected in as exact a manner as possible.
Conventional technology has predominantly used cable haulage sensors for measuring the extension length. Said cable haulage sensors typically comprise a steel cable coupled to a spring-biased winding device and a single-path or multi-path potentiometer attached thereon. Relatively recent cable-actuated encoders use contactless sensor elements, e.g. Hall sensors, instead of the potentiometers. A disadvantage of said known-actuated encoders consists in the fact that they comprise a multitude of movable parts, which in their turn are relatively susceptible to failure. In addition, their applicability in rough mobile operation is limited. Moreover, the mechanical parts exhibit increased wear so that in most cases two of said sensors are installed in order to insure sufficient safety.
A further approach known in the art to detecting the lengths of the extendable machine parts consists in providing optical distance sensors. The disadvantage of said optical distance sensors, however, consists in that light is used for measuring the distance, so that they are susceptible to contamination.
Yet other approaches describe utilization of ultrasound sensors, which are often used also in construction machines. Here, an ultrasound converter is provided which sends out sound waves having specific acceptance angles, a timer being started with said sending-out. The ultrasound waves sent out are then reflected by an object upon which they impinge and are returned to the converter, which will then act as a receiver. There the signal received is amplified, and the timer is stopped, so that the distance of the object from the converter may be inferred from the amount of time that has passed between sending and receiving the ultrasound signal. What is disadvantageous about this configuration is that the sound propagation cannot be narrowly focused, so that measuring within very small spaces, as is the case, in particular, with applications in mobile devices, is possible only with a large amount of effort.
A further approach to determining an extension length is described in DE 10 2008 048 307 A1, according to which two ultrasound sensors are provided; a first time delay between sending and receiving a first ultrasound signal sent by the first converter and a second time delay between sending and receiving a second ultrasound signal sent by the second converter are used for determining a distance. What is disadvantageous is that the system includes merely one stationary sensor and one movable sensor.
On the basis of this conventional technology, it is the object of the present invention to provide an improved approach to determining an extension length of an extendable machine part, said approach being both robust and providing a satisfactory solution to the measurement task with regard to safety, so that a system including such a device exhibits increased safety of operation and works at a higher level of precision.